NUMERICAL, ANALYTICAL AND EXPERIMENTAL STUDY OF THE THERMAL BEHAVIOUR OF INNOVATIVE ENERGY GEOSTRUCTURES

This work aims to advance understanding and knowledge of the thermal behaviour of innovative Energy Geostruc- tures (EGs), which serve a dual purpose: providing structural stability and supporting buildings or soil while harnessing geothermal energy for the heating and/or cooling demands of buildings. Two innovative EG types were investigated in this study: the Energy Quay Wall (EQW) and the Cutter Soil Mix (CSM) energy wall. The EQW integrates zeta steel sections, so-called sheet piles, embedded in soil to stabilise river or canal banks, with steel heat exchanger pipes welded onto the profiles to facilitate thermal energy exchange with both soil and open water. This study builds upon a full-scale EQW test installed in Delft, the Netherlands, which incorporated three distinct types of steel heat exchangers: (1) shallow loops and (2) add-on panels, both installed at a depth of 3 m, and (3) deep loops, positioned at a depth of 15 m. Monitoring data from the full-scale test were analysed to evaluate the energy performance of each heat ex- changer type, their impact on soil temperature, and the capability of deep loops to extract heat from open water during summer for subsurface thermal energy storage. The results reveal that the shallow loops demonstrate the highest heat extraction rate per activated quay wall surface area in most scenarios, with their performance closely linked to the open water temperature. Additionally, shallow loops do not require recharging time to restore sur- rounding temperatures, indicating stable long-term performance, yet are able to extract the least energy at the coldest time periods of the year. In contrast, the deeper loops exhibit greater stability across varying open water temperatures and achieve the highest total energy extraction per quay wall length. Based on these analyses, practi- cal charts were developed to estimate energy extraction as a function of open water and heat exchanger fluid inlet temperatures, providing a preliminary evaluation tool for assessing the thermal efficiency of EQW systems. Two 3D numerical finite element (FE) models were developed to investigate the thermal behaviour of EQWs. The first FE model, referred to as the "Thermal Initialization" model, evaluated the temperature distribution within the soil prior to the thermal activation of the EQW. The second FE model, termed the "Geothermal Activation" model, incorporated the heat exchanger pipes, which were represented in a one-dimensional framework. Both numerical FE models were validated using measured data from the full-scale test. The Geothermal activation model was subsequently employed to quantify the contributions of soil layers, open water, and air to the overall energy performance of the system. Furthermore, it was used to analyse the effects of turbulent flow within the heat exchanger pipes and the role of open water movement in optimising the thermal performance of the EQW system. The results indicate that the primary source of energy gain is from open water, and the dominance of this contribution is further increased by the presence of turbulent flow within the heat exchanger pipes. However, the soil can play a key role in short term energy delivery. Furthermore, this study emphasises the importance of the open water movement, revealing a 48% reduction in energy extraction for fully stationary water scenarios. To explore the impact of design and site-dependent parameters on extracted energy and identify strategies to enhance the energy performance of the EQW system, a sensitivity study using the Taguchi statistical method was performed. This parametric analysis was conducted using a series of 3D FE numerical models derived from the Geothermal activation model, accounting for configurations involving both steel pipes directly welded to zeta profiles (as in the case of shallow and deep loops) and add-on panels. Results show that the most influential design parameter is the number of U-loops connected in series, which can be adjusted within a reasonable range to significantly improve performance. The effects of varying inlet fluid temperature, pipe cross-sectional area, and heat exchanger fluid velocity are also critical for the energy output. Among site-specific factors, open water body temperature and its depth are confirmed as pivotal for achieving high energy performance, while new insights emerge regarding the contributions of open water flow velocity and soil thermal conductivity, particularly in short- term EQW thermal output. The second innovative EG type investigated is the CSM energy wall, constructed using the CSM technique. This approach combines in situ soil with a cementitious binder slurry to produce a mortar-like material, commonly employed for constructing cut-off and retaining walls. To enhance structural strength and resistance to bending moments, CSM walls can be reinforced with steel beams or cages, which also provide a practical support for mounting heat exchanger pipes. A full-scale CSM energy wall test site, located in Amstelveen, the Netherlands, served as the basis for this study. The thermal properties of the full-scale CSM wall were characterised through laboratory testing, then incor- porated into a 3D FE numerical model. This model replicated the full-scale test geometry, including the heat exchanger hydraulic system and soil layers, to accurately simulate the thermal exchange processes and thermal performance of the CSM energy wall full-scale test. The 3D FE numerical model was validated using data from the CSM full-scale test and applied to evaluate the short- and long-term energy performance of the CSM instal- lation under various scenarios, including insulated and uninsulated horizontal pipes and different vertical loop configurations and different energy demands. Findings indicate that energy demand significantly affects system performance, with combined heating and cooling demands enhancing long-term heat exchange rates compared to heating-only scenarios. The study demonstrates that non-insulated connection pipes increase overall heat ex- change rates, especially under heating-only conditions, while reducing the number of U-loops decreases thermal interaction and enhances energy extraction rates. After numerically evaluating the energy performance of the CSM energy wall, analytical formulations were developed to estimate one of the following parameters— outlet-inlet temperature difference, heat exchange rate, or thermal diffusivity of the wall-soil system — when the other two are known. These formulations, derived using the infinite plane source and shape factor methods, provide a simplified approach for designing the thermal system of the CSM energy wall, eliminating the need for computationally intensive numerical models. By introducing three advancements in the infinite plane source and shape factor methods, the predicted outlet temperature from the heat exchanger pipes differs by 5.7% from the measured data under steady-state conditions. These findings demonstrate the reliability and accuracy of the new analytical formulations. Overall, the conducted research facilitated significant advancements for both EQW and CSM energy wall installations by achieving the following: • Advancing the understanding of heat exchange processes governing the thermal performance of EQW and CSM systems. • Improving thermal efficiency in both short- and long-term operational scenarios. • Optimising the design process through the use of performance estimation charts for EQW systems and analytical formulations for CSM systems. These findings underscore the viability of EQW and CSM energy wall systems as sustainable and cost-efficient technologies for harnessing shallow geothermal energy.